1. Field of the Invention
The present invention relates to a battery can to be processed by drawing and ironing, a sheet for forming the battery can, and a method for manufacturing the sheet.
2. Description of the Related Arts
Normally, a cylindrical battery can comprises a closed surface, which is disposed at one end thereof and serving as a positive side and an opening portion, which is disposed at the other end thereof and serving as a negative side, on which a cover is installed. The battery is manufactured by progressive pressing method as shown in FIG. 1.
According to progressive pressing method, a coiled steel sheet S to be processed into battery cans is punched into a plurality of circular substrates M. Deep drawing is performed on each of the substrates M to form a cylindrical wall M-2 vertically on the peripheral surface of a bottom surface M-1 as shown in FIG. 1. Then, deep drawings are repeatedly performed in 8-12 processes to manufacture a cylindrical pipe to be used as a battery can having a required depth and diameter.
In the progressive pressing method, the cylindrical wall M-2 is drawn so that the thickness of the cylindrical wall M-2 becomes almost equal to that of the bottom surface M-1. Therefore, when the sheet S is drawn at 0.25 .mu.m, the thickness of the cylindrical wall of a battery can becomes approximately 0.24 .mu.m. Because the outer diameter of the battery can is regulated, preferably, the thickness of the battery can is formed to be as small as possible to make the inner diameter thereof large so that the space in the battery can is great, i.e., a greater amount of filler can be supplied to the battery can so as to increase the electric power of a battery.
In the conventional progressive pressing method, however, the thickness of the cylindrical wall and the bottom wall are reduced to the same percentage and yet it is necessary to make the thickness of the bottom wall larger than a required thickness. That is, it is difficult to reduce only the thickness of the cylindrical wall greatly. Accordingly, it is difficult for the conventional progressive pressing method to increase the electric power of the battery because a large amount of filler cannot be charged in the space of the battery. In addition, it is also difficult to reduce manufacturing cost because many processes are required to manufacture the battery can.
Drawing and ironing process method as shown in FIG. 2 has been recently developed as a method for manufacturing the battery can. According to this method, a steel sheet S is drawn and punched into substrates M in the form of shallow cylindrical cups each having a bottom wall M-1 and a cylindrical wall M-2 and each cup is processed into a cylindrical configuration having a required depth and diameter by a subsequent deep drawing process.
In deep-drawing the cup by using the above drawing and ironing process, only the cylindrical wall is drawn. Therefore, the thickness of the cylindrical wall can be reduced to 0.18 mm, whereas the thickness of the bottom wall is 0.4 mm. That is, thickness-reducing percentage is a little more than twice as great as that of conventional percentage. Therefore, a greater volume of the space of the battery cup can be provided. Thus, a greater amount of filler can be supplied into the space of the battery can. In this manner, the electric power of the battery can can be increased.
In addition, the battery can can be manufactured by only two processes. That is, the battery can be manufactured by only the process of drawing and punching the steel sheet S into cups and the process of deep-drawing the cups. Therefore, the battery can can be manufactured by a much smaller number of processes and thus at a low cost.
But the above-described deep-drawing method has the following problems. That is, in the progressive pressing method, the substrate M is drawn gradually in 8-12 processes without taking elongation coefficient (in-plane anisotropy) in lengthwise, widthwise, and oblique directions into consideration.
That is, in the above-described deep-drawing method, as shown in FIG. 3, the height of the cylindrical wall is not uniform, i.e., the highest portion A of the wall M-2 is higher by approximately 5 mm the than the lowest portion B thereof. That is, earring occurs if elongation coefficients in lengthwise, widthwise, and oblique directions are different from each other and if the thickness of a material steel sheet is not uniform. The earring is generated in cupping process and the difference between the highest portion of the wall and the lowest portion thereof becomes large in deep drawing process.
When strain is applied to a steel sheet in rolling direction (lengthwise direction X) as shown in FIG. 4 in such a manner that the steel sheet elongates within a uniform elongation coefficient, let it be supposed that the width and thickness of the steel sheet before deformation occurs in lengthwise direction X are W.sub.x0 and t.sub.x0, and the width and thickness of the steel sheet after deformation occurs in lengthwise direction X are W.sub.x and t.sub.x, anisotropy (Lankford value r.sub.x) of deformation with respect to force acting in the rolling direction is expressed by the following equation (1): EQU r.sub.x =ln (W.sub.x /W.sub.x0)/ln (t.sub.x /t.sub.x0) (1)
The Lankford value r.sub.y of force acting in widthwise direction Y and that r.sub.z of force acting in oblique direction Z forming 45.degree. with the lengthwise direction X are expressed similarly to the equation (1). In-plane anisotropy (.DELTA.r) among lengthwise direction X, widthwise Y, and oblique direction X is expressed by the following equation (2): EQU .DELTA.r=(r.sub.x +r.sub.y)/2-r.sub.z ( 2)
Experiments conducted by the present inventors indicate that the generation percentages of the earring are different from each other depending on the Lankford value r and the in-plane anisotropy .DELTA.r.
That is, the generation percentage of earring is high unless the Lankford value r is more than a predetermined value and if the absolute value of the .DELTA.r is great. Earrings are formed as follows: Projections, namely, so-called an earring is formed at four points at an interval of 90.degree. on the upper end of the cylindrical wall of the battery can. If the in-plane anisotropy .DELTA.r is positive, projections are generated at 0.degree. and 90.degree. in lengthwise direction X. If the in-plane anisotropy .DELTA.r is negative, projections are generated at 45.degree. in lengthwise direction X. When the absolute value of the in-plane anisotropy .DELTA.r approaches 0, six earrings are generated, i.e., the difference between the highest portion of the wall and the lowest portion of the wall of the battery can becomes smaller.
Supposing that the highest position of earring is A, the lowest position is B, and the required lowest position is C as shown in FIG. 3, it is necessary to cut the cylindrical wall at the lowest position B. As a result, the length between the bottom surface and the position B is short by the length between the position B and the position C.
If the steel sheet is drawn so that the length of the cylindrical wall is longer, i.e., the position B is higher than the position C, the length between the position A and the position C is greater. In this case, the sheet is wastefully consumed.
In order to prevent the generation of earring, it is necessary to make the Lankford value r in each of the lengthwise direction, the widthwise direction, and the oblique direction more than a predetermined value and in addition, make the in-plane anisotropy .DELTA.r found by the difference among the Lankford values approach zero. But it is very difficult to do so.
It is well known that when a material steel plate is rolled, the center portion thereof in the widthwise direction becomes greater and both edge portions thereof become smaller and hence the material steel plate is not rolled uniformly widthwise. When a sheet punched from the steel plate having a nonuniform thickness or a sheet punched from edge portion of such a steel sheet is deep-drawn, elongation coefficient of the sheet is nonuniform and thus the generation percentage of earring is high.
In forming the sheet into the battery can by drawing and ironing processing, cracks are likely to occur at the boundary, to be bent, between the bottom wall and the cylindrical wall of the battery can unless the sheet has a high ductility. As a result, the battery can is not corrosion-resistant.
In the above-described progressive pressing processing, the surface roughness of a sheet is high because the sheet is drawn gradually in many processes, whereas in the drawing and ironing processing, the surface roughness of the sheet is low and the surface thereof becomes smooth like a mirror surface because the thickness of the sheet is reduced to approximately 1/2 of the original thickness in two processes. That is, in the drawing and ironing processing, the inner surface of the cylindrical wall of the battery can, which contacts filler, becomes smooth like a mirror surface and thus has a high contact resistance and the battery is thus deteriorated in its electrical performance.
Preferably, the inner surface of the cylindrical wall of the battery can has a high surface roughness so as to reduce the contact resistance to increase its electrical performance, whereas preferably, the outer surface of the cylindrical wall of the battery can has a low surface roughness, namely, a smooth surface like a mirror surface because the outer surface of the cylindrical wall is corrosion-resistant and looks fine in its appearance.
As apparent from the foregoing description, in selecting the steel sheet, the most important point is property of thickness-reduction percentage namely, it is most important that the thickness thereof can be reduced at more than a predetermined percentage and the second most important requisite is that the inner surface of the cylindrical wall of the battery can has a great surface roughness affecting the electrical characteristic of the battery can and in addition, the outer surface of the cylindrical wall has a favorable corrosion-resistant performance and brightness.
The thickness-reduction percentage of the wall of the battery can, corrosion-resistant property of the inner surface of the wall, and brightness of the outer surface of the battery can are correlative to each other. More specifically, the more favorable thickness-reduction percentage is, the worse the electrical battery characteristic is, and vice versa.
More specifically, in using a sheet comprising a 5-6 .mu.m thick Fe-Ni diffusion layer formed on each surface of the steel sheet and a nickel-plated layer formed on each Fe-Ni diffusion layer, the Fe-Ni diffusion layer to be disposed on the inner surface of the cylindrical wall of the battery can is thick and hard. As a result, the nickel-plated layer are cracked like wedges in drawing and ironing processing. The cracks affects favorably in the electrical performance of the battery can because the contact resistance of the inner surface to filler which is supplied into the battery can and is brought into contact with the inner surface is low. But the sheet cannot be drawn favorably, i.e., earring is generated on the upper end of the wall of the battery can. In addition, because the outer surface of the wall of the battery can is as thick as 5-6 .mu.m, cracks are likely to be generated on the outer surface in drawing and ironing processing.
The conventional steel sheet to be processed into the battery can by the drawing and ironing processing comprises a low carbon steel containing carbon at 0.04-0.05 wt. %. The steel sheet is manufactured by the method as shown in FIG. 5.
That is, at step #1 slab is hot-rolled, and at step #2, the hot-rolled steel plate is cold-rolled. At step #3, the non-annealed cold-rolled steel plate is batch-annealed, and at step #4, refining rolling is carried out. A step #5, the upper and lower surfaces of the steel plate is plated. Thereafter, continuous annealing and refining rolling are performed on the plated steel plate at step #6 and step #7, respectively. Then, the steel plate is plated at step #8.
In the above conventional manufacturing method, however, batch annealing (step #3) has the following disadvantages:
In the batch annealing, steel sheets S' are annealed in an annealing oven w by piling them one on the other in a hoop configuration. Therefore, in order to prevent them from being coalesced with each other and thus damaged, it is necessary to apply sodium silicate to the surface of each steel sheet S' to form a film thereon. But when the film of sodium silicate has been broken, the film becomes powders and in addition, iron powders are generated, thus causing the surface of the steel sheet to deteriorate. In particular, when iron powders have adhered to the surface of a roll for rolling the steel sheet (step #4), iron powders are transferred to the steel sheet.
In addition, as shown in FIG. 7, the steel sheet S' is heated at 550.degree.-600.degree. C. for an hour and the temperature is maintained at 550.degree.-600.degree. C. for 2-10 hours. Then, the steel sheet S' is gradually cooled to 100.degree. C. in 23-33 hours. Therefore, the period of time required from the start of annealing until the take-out of the steel sheet S' from the oven W is as long as about 36 hours, which causes manufacturing cost to be high.
Further, because the steel sheets S' are annealed in a hoop configuration, heat is not uniformly distributed to the steel sheet S' and hence it is difficult to anneal them uniformly.
In the continuous annealing to be performed at step #6, the steel sheet is annealed while it is being transported by rolls. Therefore, it is unnecessary to apply sodium silicate to the surface of the steel sheet S' to form a film thereon and heat is uniformly distributed to the steel plate S'. In the continuous annealing, as shown in FIG. 8, the steel sheet S' is rapidly heated to 600.degree.-900.degree. C. for a minute, and the temperature is maintained at 600.degree.-900.degree. C. for 30 seconds. Then, the steel sheet is cooled to 400.degree. C. in 20 seconds, and then, over-aging processing is performed for 150 seconds. Thereafter, the steel sheet S' is rapidly cooled to 100.degree. C. in 15 seconds. Thus, the period of time required from the start of the continuous annealing until the take-out of the steel sheet S' from the oven W is as short as about five minutes.
Continuous annealing cannot be performed in the case of low carbon steel containing carbon at 0.04-0.05 wt. % at step #3 instead of batch processing for the following reason:
That is, at the room temperature, .alpha. solid solution (ferrite) and Fe.sub.3 C (pearlite) are in a mixed state in the low carbon steel. When the low carbon steel has not been annealed, cementite (carbon atoms A) in ferrite tends to concentrate on a dislocation portion D as shown in FIG. 9. When an external force is applied to the steel plate, the dislocation portion D moves along a sliding surface X. As a result, resistance generated by cementitc is added to the resistance to the movement of the dislocation portion D, so that cementite has concentrated on the dislocation portion D. Consequently, yield point elongation Y1 is generated as shown in FIG. 10A. Stretcher strains are generated on the surfaces of the battery can 1' as shown in FIG. 11 when a steel sheet having the yield point elongation Y1 is drawn and ironed. In particular, because the top surface 1a' of the battery can 1' is exposed to the outside, the presence of the stretcher strain on the top surface 1a' of the battery can 1' is commercially defective.
When the low carbon steel is heated, cementite becomes a super-saturated solid solution and thus re-solidified. Therefore, the yield point elongation Y1 disappears as shown in FIG. 10B when annealing and refining rolling are carried out. The steel plate is heated rapidly and cooled in continuous annealing as described above. Accordingly, strain is not completely removed from the steel plate by re-crystallization. Consequently, the yield point elongation Y1 (time-aging) is generated again as shown in FIG. 10C with the elapse of time. In batch annealing, because the steel sheet is gradually heated and cooled and thus crystal having no strain is formed namely, aging does not occur. For the reason described above, in order to prevent the generation of stretcher strain, continuous annealing is inappropriate for annealing the cold-rolled plate. Thus, batch annealing cannot but be adopted for low carbon steel containing carbon at 0.04-0.05 wt. %.
The above-described drawing and ironing processing has the following disadvantage because the thickness of the steel sheet is reduced to approximately 1/2 of the original thickness thereof by a die shown in FIG. 12: That is, the friction coefficient of the peripheral surface 51a of a cup 51 made of a sheet to be processed into the battery can 1 and that of the surface 52a of a die 52 which slidingly contacts the peripheral surface 51a are very great. Similarly, the friction coefficient of the inner surface 51b of the cup 51 and that of the outer surface 53a of a punch 53 which slidingly contacts the inner surface 51b are very great.
Accordingly, the longevity of the die 52 and that of the punch 53 become very short. In addition, a great force is required to separate the punch 53 and the punched battery can from each other.
Normally, lubricating oil is applied to the upper and lower surfaces of a sheet to be processed into the battery can before a blanking processing is carried out. Because the upper and lower surfaces of the sheet is smooth, lubricating oil is apt to drop therefrom and often decreases or is exhausted before or when the drawing and ironing processing is carried out after the blanking processing and cupping processing are performed.
Lubricating oil may be supplied to the periphery of the hole of the die and the outer surface of the punch. But because the thickness of the material sheet is reduced to a great extent, lubricating oil is exhausted soon. As a result, it is necessary to supply lubricating oil frequently to the periphery of the hole of the die and the outer surface of the punch. That is, operational performance is low.